PROJECT SUMMARY/ABSTRACT The information that reaches our eyes doesn?t always correspond to what we perceive. Attention strongly influences what we see and how we interpret the world. At the same time, our daily activities often require us to divide attention across two or more things. Previous studies quantifying how much divided attention degrades task performance have produced varying results, and the neural bases for these effects are poorly understood. My proposal builds on recent studies of divided attention to masked words, which were consistent with an attentional bottleneck (only one word can be processed at a time). Specifically, I will pursue the hypothesis that divided attention effects in object recognition are due to an attentional bottleneck. I plan to use a combination of theory-driven psychophysics and functional neuroimaging to investigate (1) how much divided attention impairs human ability to recognize objects, (2) how much divided attention changes neural activity, and where in the brain this takes place, and (3) whether divided attention can explain why patients with posterior cortical atrophy (a variant of Alzheimer?s disease affecting visual areas) exhibit deficits in recognizing simultaneously presented objects. Studying vision and attention together can have powerful implications for our understanding of the relationship between brain and behavior. My findings will make an impact by informing theories of how cognitive states relate to brain activity, and how these changes underlie the limits in our ability to make perceptual judgments. Beyond the benefit to advancing the fields of visual and attentional processing, the proposed research will also make an impact in understanding conditions involving deficits in perceptual and attentional processing functions, including aging and dementias.